1. Field of the Invention
The present invention relates to an ink, particularly an ink jet black ink that performs recording on a recording medium by being ejected from an orifice in accordance with a recording signal and can be used suitably for ink jet recording, an ink set, an ink jet recording method, an ink cartridge, a recording unit, and an ink jet recording apparatus.
2. Related Background Art
An ink jet recording method is coming to be widely used particularly in an office or for personal use as means for easily forming color images. In recent years, images recorded by ink jet recording has a higher image quality, so that it is possible to realize recorded images having a high image quality even comparable to that of a silver halide photograph. For this reason, there are requests for performance such as excellency in color tone reproducibility, product quality performance on the recorded images such as giving images with high optical density and a vivid color tone, and further a long-term storage property of the images. The long-term storage property is namely an image fastness such as a property that the images do not undergo color change or color degradation due to sunlight or various illuminations (light resistance), or that the images do not undergo color change or color degradation by oxidizing gas (ozone, NOx, SOx, or the like) contained in a slight amount in environmental atmosphere (gas resistance).
Also, in recent years, as a recording medium for ink jet, a recording medium is used in which a porous material such as alumina or silica is used in an ink receiving material so as to obtain an image quality comparable to or surpassing that of a silver halide photograph. In particular, a recording medium that uses alumina hydrate has advantages in that, because alumina hydrate has a positive electric charge, the coloring material having a negative electric charge in ink is fixed well to give an image with good color generation, and that the recording medium is preferable when compared with a conventional recording medium particularly in terms of image quality and gloss in full-color images.
Regarding the above-described fastness, the aforesaid recording medium may in some cases exhibit a considerable color change or degradation in terms of gas resistance, so that an improvement is desired.
Also, regarding black ink, the following are particularly desired in terms of image performance and fastness.
First, with regard to the image performance, it is desired to enhance the optical density and make the color tone be near to the neutral, so that various proposals have been made. For example, a technique with an ink using specific two kinds of dyes is shown (For example, see Japanese Patent Application Laid-open No. 2000-290552). Also, a technique is disclosed in which, after a treating agent containing a polymer substance is applied to a recording medium, an ink containing a coloring material is made to adhere to the recording medium to aggregate the coloring material, thereby improving the OD (optical density) as a result (For example, see Japanese Patent Application Laid-open No. 2000-318300). Also, as a dye that exhibits a high optical density, an ink that uses a novel dis-azo or tris-azo dye is disclosed (For example, see Japanese Patent Application Laid-open No. 2002-275380 and Japanese Patent Application Laid-open No. 2003-292808).
Next, with regard to the fastness, there is a proposal to improve the color tone and the color change resistance against ozone gas (For example, see Japanese Patent No. 02919615 and Japanese Patent Application Publication No. H08-26263). However, in the ink jet recording method of recent years that can give recorded images of high image quality comparable to that of a silver halide photograph, a further improvement of gas resistance is desired. Above all, the fastness tends to become more aggravated in images that are a little sparse, i.e. in a half tone, than in images having a large amount of applied ink, so that the degradation of fastness is conspicuous in recorded objects. Therefore, it is extremely important to maintain the fastness to be good even in a part having a small amount of applied ink.
From the past, it is generally known that a pigment is superior to a dye in terms of fastness, so that many proposals of an ink using a pigment have been made. When a pigment ink is used, the fastness is good when printed on an ordinary paper sheet or on a recording medium of a so-called swelling type that is provided with a resin of swelling type in an ink receiving layer. However, when the ink is printed on a recording medium of a so-called absorbing type that is provided with a porous material in an ink receiving layer, the pigment does not permeate into the inside of the recording medium and is aggregated on the recording medium because the pore of the porous material is small as compared with the particle size of the pigment in the ink. This generates a metal luster (bronze) and may in some cases lead to decrease in the optical density.
With regard to an ink that uses a pigment, a combined use of a pigment and a dye is proposed (For example, see Japanese Patent Application Publication Nos. H07-68480, H07-68481, H07-68482, Japanese Patent Application Laid-open Nos. H11-240145 and H11-286126). However, these proposals aim at making improvements with respect to an ordinary paper sheet or a recording medium having a resin layer as a coating layer.